Inks for ink jet systems have been traditionally water-based compositions. There have been many problems associated with water-based inks. These problems include nozzle clogging, nozzle crusting, alterations in surface tension due to repeated exposure to the atmosphere during the jetting and recirculation of the ink fluid, print quality (including optical density and dot characteristics), and lack of adhesion of ink to the substrate. Many of these problems may be inter-related, e.g., alterations in surface properties of an ink caused by repeated exposure to the atmosphere during jetting and recirculation may cause nozzle clogging. Evaporation of a portion of the base liquid or absorbance of atmospheric moisture by the ink fluid may, for example, change the solubility of a particular dye in the ink fluid.
U.S. Pat. No. 3,994,736, discloses the use of polyhydric alcohols such as propylene glycol and ethylene glycol in the base liquid because of their ability to prevent the drying of the ink in the nozzle with subsequent clogging of the nozzle orifice. The polyhydric alcohols may be mixed with a lower monohydric alcohol to ensure rapid drying of the ink after deposition on the substrate to be printed.
Even with the addition of humectants to solve the drying problems, water-based inks are still troublesome because of the hydrophobic nature of most printing papers. The addition of surfactants to these inks, while improving the absorption of the inks into the paper, often severely affect the surface tension and the jetability of the ink fluid. Therefore, the printing quality of these inks is never any bettter than marginal as compared with impact printing quality.
Therefore, attention was directed towards the use of dye materials which are physically more compatible with printing paper.
A fluid medium was selected from the typewriter art, e.g. oleic acid. Oleic acid is known to provide good print quality, because it is easily absorbed into most printing papers. There are problems with the use of oleic acid as an ink jet substance, however. The oleic acid has too high a viscosity for good jetability. While additives can be used to improve the viscosity, the necessary surface tension of the ink fluid will usually fall below acceptable levels.
The present invention teaches how to properly formulate and use oleic acid as a fluid medium for ink jet printing. Part of the solution to the problem of jetability is to utilize the oleic acid compositions at elevated temperatures, generally in a range from 125.degree. F. to 145.degree. F. While it may have been suggested to use water-based ink jet inks at elevated printing temperatures, the use of oil based inks, and particularly, the use of oleic acid-based ink jet inks at elevated temperatures has never been described or suggested in the art. Neither has it been taught that ink jet ink formulations containing oleic acid can provide superior print quality as described herein.